Filamentary cathode for electron tubes



Oct. 10, 1950 H. E. soRG ETAL 2,525,565 FILAMENTARY CATHODE FOR ELECTRON mass Fil'qd Jply 12, 1948 INVENTORS HAR LD E. 5026 PAUL 0. W/L L/A M5 DONALD E Dk/ESCHMAN BY flaw/5% ATTORNEY Patented Oct. 10, 1950 FILAMENTARY CATHODE FOR ELECTRON TUBES Harold E. Sorg, Redwood City, Paul D. Williams, Palo Alto, and Donald F. Drieschman, Los Altos, Calif., assignors to Eitel-McCullough, Inc., San Bruno, Calif., a corporation of California Application July 12, 1948, Serial No. 38,338

3 Claims.

Our present invention relates to electron emitters for electron tubes of the character disclosed in our copending application, Serial No. 38,114, filed July 10, 1948, and more particularly to improvements in filamentary type cathodes.

Thoriated tungsten wire (tungsten containing around 1% thoria) has long been used for filaments in electron tubes, particularly for power or transmitting type tubes, the practice being to form the wire into a helix or other suitable shape, mount the filament on a stem, and carburize the filament. The purpose of carburizing the filament is to improve the electron emission properties of the thoriated tungsten, the carbon (present as tungsten carbide) providing a reducing agent for the thoria during operation of the electron tube.

Several important problems exist in the use of carburized thoriated tungsten wire as a filament in electron tubes. One of the worst problems is that such filaments are very brittle and much loss occurs during transportation and tube operation due to broken filaments. Another problem is that such filaments tend to lose their carbon at excessive rates in certain cases, resulting in short tube lift. In the electrone tube art this loss of carbon from the filament is called the decarburization rate and is one of the controlling factors determining the electron emission life of the filament and hence the life of the tube.

It is among the objects of our invention to provide a carburized thoriated tungsten type of filament which is less brittle and able to withstand quite severe shocks without breakage.

Another object is to improve the electron emission life of such filaments.

The invention possesses other objects and features of advantage, some of which, with the foregoing, will be set forth in the following description of our invention. It is to be understood that we do not limit ourselves to this disclosure of species of our invention as we may adopt variant embodiments thereof within the scope of the claims.

Referring to the drawing:

Figure 1 is an elevational view of an electron tube embodying the improvements of our invention, portions of the internal parts being broken away to show the structure more clearly.

Figure 2 is a detail view showing the filamentary cathode, and

Figure 3 is an enlarged view showing a portion of the improved filament wire.

In terms of broad inclusion, our improved filament material comprises a wire core, and an outer layer on the core comprising .convolutions of. In its final form as a cathode such outer layer comprises the electron,

emitter layer and is carburized. At least apart,

thoriated tungsten wire.

of the metal in the core is also preferably carburized. A core Wire of tungsten-tantalum is preferred. 1

In greater detail and referring first to Figures 1 and 2 of the drawing, a triode type of internal anode tube is chosen for purposes of illustration,

it being understood that the improved filamentary.

cathode of our invention may be used in many other types of electron tubes. tr-ated comprises an evacuated glass envelope '2 with a stem 3 carrying an exhaust tubulation t and provided with a base 6 having terminal prongs 'l. The envelope encloses a filamentary cathode B hereinafter described in detail, a tu -v bular metal anode 9 and a cage-type wire grid I I. Anode 9 has a cap [2 supported by a bracketl3 on lead l4 sealed to the upper end of the glass.

envelope.

A pair of leads I! sealed to stem 3 support the cathode, which leads are connected to apair of the base prongs l by conductors I8. The grid ll has a base ring 2| supported by brackets 22 on a pair of rods 23 sealed to stem-3, one of these rods being connected bya conductor 24 to a base: Figure 2 shows the filament 8 mOrecIearly, the particular shape of filament chosen-for purposes of illustration being helical and con- Again it is understood that the helical shape is merely il' prong.

nected at its ends to the leads l'l.

lustrative, as many other filament'formation's,

including hairpin types and the like, may be used.

Referring now to Figure 3, our improved filament material comprises a core wire'26 carryingan outer layer 27 comprising close convolutions; of thoriated tungsten wire. The wire used for thewrapping is preferably a commercial grade' of thoriated tungsten wire containing about 1% thoria such as commonly used for making electron tube filaments. The core-Wire 26 comprises a metal having a relatively high melting point and low vapor pressure such as tungsten, tantalum or molybdenum, or a combination of such ing desirable, however, that the percentage 9;"

alloy.

The tube illustantalum be larger. Such an alloy has good strength yet is reasonably ductile and is well suited as a core wire in our improved filament material.

The wire sizes used for the core 26 and outer winding 21 may be varied within wide limits, depending upon the overall size of filament wire desired. In any case, the thoriated tungsten wire used for the outer layer is preferably smaller than the core wire. For example, with a core wire of say 10 mils diameter a thoriated tungsten wire of about 3 mils diameter is satisfactory. The thoriated tungsten wire in layer 21 is preferably of circular cross-section but a flattened wire or strip may be used. The wrapped layer 21 may be applied by a suitable winding machine, it being desirable to heat the thoriated tungsten wire as it is applied so that when cooled it will shrink tightly against the core. If desired, the layer 21 may be further sintered to the core by applying a coating of finely divided tungsten powder on the core before the winding is applied. Subsequent heating of the filament wire then fuses the fine particles to provide a sintered bond.

The composite filament Wire above described is now ready for shaping into the desired filament formation such as the helical shape shown in Figure 2. Mounting of the filament on the stem may be done in accordance with conventional practice, as by welding the ends of the filament to its supporting leads H.

The thoriated tungsten layer 21 is then carburized. This is preferably accomplished by heating the filament to about 2100 C. brightness temperature in an atmosphere containing a hydrocarbon gas such as methane. Placing the stem in a bell jar containing the gas and heating the filament by passing current through the leads 1'! is a satisfactory procedure. At the above temperature the thoriated tungsten layer takes carbon from the atmosphere, converting at least a part of the tungsten in the outer portions of the layer 21 to tungsten carbide. The depth of penetration of the tungsten carbide in the layer 2'! can be controlled by the temperature and time of heating in the hydrocarbon atmosphere. This carburized thoriated tungsten outer layer 21 constitutes the electron emitter layer of our filament, and the fact that this layer is carried by a separate core wire 26 has several important advantages hereinafter discussed. After carburization the filament is ready for sealing in the tube envelope and from this point on the tube is handled and evacuated by known procedures used in making tubes with thoriated tungsten type electron emitters.

Our improved filamentary cathode is decidedly stronger than ordinary carburized thoriated tungsten filaments. The reason for this improvement isthat the core wire 26 functions as a supporting strand and is of a metal particularly selected for its strength properties. Comparative tests made with a standard high impact machine supplied by the Navy Department show that our improved filaments will withstand shocks of several hundred gs more than ordinary carburized thoriated tungsten filaments.

Still another important advantage of our improved filament, due to the presence of the core wire 26, is that a reserve supply of carbon can be stored in the core for supplying carbon to the thoriated tungsten layer 21 during the life of the filament. To achieve this result the core wire 26 is carburized, at least in part, to provide a reservoir of carbon underlying the emitter layer 21. The preferred procedure in this case is to carburize the outer surface of the core wire, as by heating the core wire in a hydrocarbon atmosphere, before the wrapping of thoriated tungsten wire is applied. The carbide region underlying the thoriated tungsten layer 21 thus provides a source of carbon which is available for replenishing the carbon in the adjacent thoriated tun sten layer in event thoriated tungsten tend to lose its carbon too rapidly during operation of the filament. A longer life filament is thus insured. We have found that such carburization of a tungsten-tantalum alloy core, which converts at least a part of the tungsten phase of the alloy to tungsten carbide, does not materially reduce the strength properties of the core wire.

We claim:

1. A filamentary cathode for an electron tube comprising a wire core of tungsten-tantalum al- 1037, and an emitter layer comprising convolutions of carburized thoriated tungsten wire on said wire core.

2. A filamentary cathode for an electron tube comprising a wire core of an alloy containing tungsten and tantalum, at least a part of the tungsten being carburized, and an emitter layer comprising convolutions of carburized thoriated tungsten wire on said wire core.

3. A filament material for an electron tube comprising a wire core of tungsten-tantalum alloy, and an outer layer on the core comprising convolutions of thoriated tungsten wire.

HAROLD E. SORG. PAUL D. WILLIAMS. DONALD F. DRIESCHMAN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,731,244 Gero Oct. 15, 1929 1,731,255 Marden Oct. 15, 1929 2,297,454 Berger Sept. 29, 1942 FOREIGN PATENTS Number Country Date 618,580 France Dec. 20, 1926 

1. A FLAMENTARY CATHODE FOR AN ELECTRON TUBE COMPRISING A WIRE CORE OF TUNGSTEN-TANTALUM ALLOY, AND AN EMITTER LAYER COMPRISING CONVOLUTIONS OF CARBURIZED THORIATED KTUNGSTEN WIRE ON SAID WIRE CORE. 